with that anxious wait. World leaders wondering if and when north korea will make a move. After weeks of escalating threats, some believe a missile launch could come at any moment. It is already monday morning in north korea. And they are celebrating the birthday of that country's founder, kim il-sung. That's him on the left there. His grandson, kim jong-un on the right. And he is the young and untested leader behind the escalating rhetoric and tensions with the south. this image, mocking the grandfather and grandson, after north korea rejected a new offer from the south to ease tensions. Abc's bob woodruff, standing in seoul, south korea, again tonight. Reporter: For secretary of state john kerry, japan is the last and arguably most tense leg of his asian tour. While he spent some of his time visiting public sites, the bulk of the day was about solving the crisis with north korea. The biggest priority is a peaceful resolution to the issues of north korea. Reporter: Kerry's japan visit puts him well within the range of a threatened missile test from north korea. A launch that could come at any moment, as the north marks its biggest holiday, the birth of founder kim il-sung. The missile, known as musadan, has never been fired before. The most likely trajectory would send it over japan, towards the pacific ocean. The u.S. Has put more patriot missile interceptors in key positions and has ships and satellite surveice across the region, just in case. Perhaps the most important meeting of the weekend was in china, north korea's only real ally. Secretary kerry is trying to get china to put some teeth into their efforts to persuade kim jong-un to stop developing nuclear weapons. But it's not clear how much pressure china is willing to apply. Despite the tensions, signs of normalcy on both sides of the border. In pyongyang, they ran an international marathon this weekend. And in seoul, music star psy streamed a concert on youtube, which drew record viewers. Life going on as usual, under the military threat. Kim jong-un has not made any public appearances since april 1st. That's two weeks ago. Also today, the north rejected an offer from the south to talk, saying that this is a cunning ploy, and empty promises. That's more rhetoric. But we don't know exactly when or if this launch will happen today.

This transcript has been automatically generated and may not be 100% accurate.